we've got one but the vrt 340f has a dial on as you say.
Turning the dial towards the + increases the number on display to the temp required.
have you tried pressing the + on the controller to see whether the value increases?
If so keep doing it until you see the radiator symbol.

Yep.. I've pressed plus and it shows 18 or 19.. until it gets to 20 and then goes no further. When i'm doing this the hand icon shows on the display (exactly the same as the one shown on the front of the boiler)

Looks like the system s set to water only at the moment
We have a switch n the boiler that sets the heating to winter mode, water and heating
The wireless icon only shows when the unit sends a signal to the boiler
When using the + and - buttons to set the temp don't forget to press ok to confirm the temp

We used to have a simpler version of the thermostat in our old house. The wireless kept dropping the link and disconnecting the boiler from the thermostat. And the temperature never matched what was going on in the rooms.

Our solution was to crank the thermostat to MAX, so that it was always on, then set the boiler on times using the clock timer on the boiler.

One more thought - the 26 deg on the boiler might be the water output temperature in the radiator system. Ours is currently at 65 deg. If you want a room at 20 deg and the radiators only get to 26 deg you wont be putting enough heat in to get it hot.

Try pressing the -+ buttons on the boiler together for a few seconds. It should say sfh and give you some heat for a few minutes. That'll show if it is working and that the problem is probably the thermostat/controls

Well I've looked at those instructions for 20 seconds and your room stat looks to be set to water heating @ 19°C. You need to find the boiler instructions and then RTFM's. As said, boilers for heat and water are often set to max but you need to confirm this via the book of words.

I'd set the stat to run the heating constantly (the ladder icon) then work out how to switch the boiler on. That way you take the stat out of the equation and the stat instructions linked look reasonably easy to follow.